Device for treating wafers on assembly carriers

ABSTRACT

A device for treating wafers on assembly carriers is disclosed. A wafer to be treated can be fixed on a liquid film that is located between the front side of the wafer and the assembly carrier by freezing of the film.

This application is a divisional of application Ser. No. 11/852,885filed on Sep. 10, 2007, now U.S. Pat. No. 7,918,714 which was acontinuation of co-pending International Application No.PCT/EP2006/050783, filed Feb. 8, 2006, which designated the UnitedStates and was not published in English, and which is based on GermanApplication No. 10 2005 011 107.6 filed Mar. 10, 2005, all three ofwhich applications are incorporated herein by reference.

TECHNICAL FIELD

An embodiment of the present invention relates to a device for treatingwafers on assembly carriers, for example, for fixing and releasingwafers on assembly carriers.

BACKGROUND

For thinning the wafers before certain process steps, such as forexample sawing or loading with components, the substrates are ground onthe back side. A method of doing this that is known from practical useis one in which the front sides of the wafers are laminated with aprotective film. The wafer is fixed with the laminated front side on thevacuum grinding table (vacuum chuck) and thinned by means ofdiamond-impregnated grinding wheels while cooling water is additionallyapplied.

The protective film is also intended to equalize and even outprocess-dependent differences in height in the circuits on the wafer.Pronounced topographies in the semiconductor circuit or pronouncedsubstrate thinnings cause local nonuniform variations in the thicknessof the substrate (dimples) during the grinding. Resultant mechanicalstresses in the substrate can lead to cracks and chip rupture. Inparticular, in the case of more recent forms of flip-chip mounting,metal connectors (bumps) with a height of up to 150 μm are additionallyapplied on the front side, serving for direct contacting on printedcircuit boards. These high bumps can no longer be smoothly covered withconventional protective films. Back-side thinning below a substratethickness of 300 μm without any deterioration is problematic.

Topographies on the wafer with a height of up to 50 μm can be equalizedby means of thicker layers of adhesive on the protective films. However,possible remains of adhesive, and consequently increased contamination,pose a risk to the reliability of component mounting operations. Eveningout by means of additional layers of lacquer on the surface of thewafer, and in particular their removal/disposal, make the processconsiderably more expensive. For even higher contact bumps, specialfilms are being developed; the price of such films is twice that of theexisting standard material.

The “protective film lamination” and “protective film removal” processesrequire dedicated devices of their own. In the laminating device, theprotective film is adhesively attached to the upper side of the waferand cut-off flush at the periphery of the wafer by a heated kniferunning around it. Thinning of the back side by grinding is followed inthe peeling device by fixing the wafer on a vacuum chuck, adhesivelyattaching a special peeling adhesive tape and peeling off the protectivefilm from the wafer while supplying ionized air (to counteractelectrostatic charging during the peeling off of the film). These methodsteps are laborious.

Previously, the grinding thin of the wafers had to be performed beforeapplying the contact bumps, which involved increased risk of rupture ofthe thin wafers in the subsequent metallizing and etching processes.

Various methods and devices that are concerned with the problems offixing wafers on assembly carriers are also known from the patentliterature. Some of these address the difficulties of fixing wafers onassembly carriers when the wafers are to be thinned.

One example of such a literature reference is European Patentpublication, EP 1 148 554 A1, U.S. counterpart U.S. Pat. No. 6,683,332B2. Described there is a method for the highly accurate and rapidthinning of a semiconductor substrate, which can be performedindependently of the tolerance of an assembly carrier and an adhesivejoin with which the substrate is attached to the assembly carrier. Forthis purpose, a first doped layer with p dopant is formed in thesubstrate. Subsequently, the substrate is initially ground down from itsback side and further etched back wet-chemically. In this case, thefirst doped layer serves as an etching resist. In this document, thefollowing literature reference is cited by way of example as prior art:“Semiconductor Wafer Bonding: Science and Technology, Q. Y. Tong,Wiley-Interscience Publication”. There it is described on pages 1 to 13that, in the case of such a method, the process of thinning thesemiconductor substrate is one of the technologically most demanding andexpensive process steps. For the three-dimensional integration, usuallytwo ready-processed wafers are first provided. The first wafer serves inthis case as a carrier, the second wafer is thinned by the followingmethod and arranged on the first wafer. For thinning, the second waferis first provided with a layer of adhesive on its front side, which isthe side with the electric circuits, and is then connected to anassembly carrier. The second wafer is then thinned from its back side,usually up to three methods being used sequentially.

The first method to be used is usually a grinding method, which isfollowed by a chemical etching method and chemical-mechanical polishing(CMP). The aim of this method is to retain a residual thickness of thesemiconductor substrate in the range of 10 μm, special importance havingto be attached to the planarity and the exact maintenance of the targetthickness. On account of the different ways in which they work, thethree thinning methods mentioned each entail different disadvantages, sothat the best result is achieved by a combination of the known methods.Grinding is the quickest method, and is therefore used as the firststep, in order to remove the greatest part of the semiconductor layer.

However, grinding causes deteriorations of the substrate surface, whichare removed in a subsequent chemical etching step. The chemical etchingstep has the disadvantage, however, that the etched surface is notplanar but has a waviness in the range of +/−3%, the layer thicknessremoved by the etching step. For this reason, chemical-mechanicalpolishing CMP is carried out in a third step, whereby the waviness ofthe surface is polished out. The CMP step is slow and expensive and istherefore only used for the post-treatment of the surface. Themechanical grinding is used as the method with the greatest removal. Theadjustment of the installation means that the removal during grindingoccurs plane-parallel to the assembly carrier to which the secondsubstrate wafer is attached.

Here it must be taken into consideration that a wafer that is notattached plane-parallel to the assembly carrier is ground awayobliquely. Since the substrate wafer is attached to the assemblycarrier, for example, by adhesive, there is an adhesive join between thesubstrate and the assembly carrier. If the adhesive join has a differentlayer thickness, as formed for example in the case of a wedge shape, thesubstrate is not aligned plane-parallel to the assembly carrier. In thesubsequent grinding process, the substrate wafer is therefore not groundaway plane-parallel to the surface in which the electric circuits arearranged. This problem can be solved, for example, by the adhesive joinbeing made very thin. However, this has the disadvantage that no filledadhesives can be used, adhesives which would be advantageous during thelater detachment of the substrate from the assembly carrier, since, forexample, solvent can remove the adhesive more easily from thick adhesivejoins. Likewise, the adjusting accuracy of the assembly carrier withrespect to the grinding plate is transferred into the accuracy of thegrinding process. On the other hand, it is not possible to dispense withthe grinding process, since etching processes are too inaccurate and CMPis too slow.

There is, for example, a known method in which a buried oxide layer isused as an etching resist. Wafers which have such a buried oxide layerare known as SOI (silicon on insulator) wafers. These wafers are muchmore expensive than standard wafers and require a modified processprocedure in the production of circuits in the silicon substrate ascompared with conventional silicon wafers. This necessitates anadaptation of the process technology. Particularly disadvantageous inthe case of SOI wafers is that they have great internal mechanicalstresses. If SOI wafers are thinned to a few 10 μm and below, this leadsto the silicon layer peeling off from the assembly carrier and to thesilicon layer rolling up.

Devices that work differently are also known. For example, EuropeanPatent publication, EP 0 737 546 B1, U.S. counterpart U.S. Pat. No.5,791,973, specifies as prior art a device which serves for thechemical-mechanical polishing of semiconductor substrates. As alreadymentioned, chemical-mechanical polishing is also referred to by theabbreviation CMP, which will be used hereafter.

In the cited documents, a description is given of a device which has aturntable that comprises a planar surface and consists of stablematerial, so that it is of a sufficiently rigid configuration. By meansof the driven rotary shaft, the turntable is turned in a predeterminabledirection. On the surface of the turntable there is a polishing padand/or a polishing material. Above the turntable there is a holding headfor the substrate to be polished, for example, a semiconductor wafer,referred to hereafter as wafer for short. The substrate holding head maylikewise be formed in a rotatable manner. With the aid of the substrateholding head, the substrate to be polished is pressed against thepolishing pad and polished on account of the relative rotationalmovements under the action of the abrasive material. The polishingoperation takes place under certain contact pressure, and it may happenthat the substrate to be polished becomes detached from the substrateholding head and moves over the turntable in an uncontrolled manner.This can cause damage to the device. If this happens, the substrate inany case becomes unusable.

SUMMARY OF THE INVENTION

In one aspect, the invention provides a method for treating wafers onassembly carriers, for example, for fixing and releasing wafers, and adevice for carrying out the method. The wafer is handled with extremecare and the device for fixing and releasing the wafer is of a simpleconstruction and it is possible for the method to be carried outreliably and easily.

In one embodiment, a method for treating wafers on assembly carriers isdisclosed. For example, wafers can be fixed and released on the frontsides of which electric circuits are arranged and the back sides ofwhich are subjected to treatment steps. In a first embodiment, anassembly carrier is provided. The surface of the assembly carrier thatis facing the wafer to be treated is wetted with a liquid film. Thewafer to be treated is placed with its front side onto the liquid film.The liquid film is cooled until at least, or only, its peripheral zoneor the entire contact surface solidifies thereby fixing the wafer to theassembly carrier. The back side of the wafer is treated. The solidifiedportions or zones of the liquid film is heated until they liquefy. Thetreated wafer is removed from the assembly carrier.

It is in this case advantageous if the cooling of the liquid film isperformed by liquid nitrogen and if the liquid nitrogen is fed to theassembly carrier.

The liquid nitrogen may in this case be fed to the assembly carrier viathe wafer.

It is also advantageous if the liquid nitrogen serves as a coolant andlubricant during the back-side treatment of the wafer.

The liquid nitrogen may, however, also be fed to the assembly carrier bychannels.

Furthermore, it is an advantage that, as a result of the region of theliquid film that is kept liquid within the solidified peripheral zone,the front side of the wafer, provided with electric circuits, isprotected from mechanical pressure peaks and vibrations during theback-side treatment of the wafer.

Furthermore, it is advantageous that, as a result of the solidifying ofthe peripheral zone of the liquid film, the penetration of foreignsubstances is prevented and contamination of the wafer and the assemblycarrier is prevented.

In addition, it is an advantage that, as a result of the region of theliquid film that is kept liquid within the solidified peripheral zone,small particles which may be located on the assembly carrier or on thewafer do not damage the sensitive front side of the wafer that isprovided with electric circuits, since they are embedded in liquid.

Furthermore, it is advantageous that, as a result of the cooling of theback side of the wafer, the oxidation on its surface is kept low duringthe treatment process.

It is particularly advantageous if at least one of the treatment stepsis the grinding thin of the wafer, in particular, if it is the grindingthin of the back side of the wafer.

Particularly suitable for carrying out the method with the advantagesdescribed above is a device for fixing wafers on assembly carriers, inparticular wafers of which the back sides are subjected to treatmentsteps, when there is a liquid film on the assembly carrier, in which thefront side of the wafer can be embedded, and when furthermore there is atemperature-controlling device for controlling the temperature of theliquid film, it being possible by means of the temperature-controllingdevice for the state of aggregation of the liquid film to be partiallychanged in such a way that an annular peripheral zone of the liquid filmcan be made to solidify and the wafer can be fixed to the solidifiedperipheral zone.

Also of advantage is a device in which the assembly carrier comprises avacuum table which has a ceramic element for receiving the wafer, andthere is a liquid film between the ceramic element and the wafer.

A device is also favorable if there are channels in the peripheral zoneof the ceramic element, and if media with the aid of which the state ofaggregation of the liquid film can be changed flow in the channels, itbeing possible for the peripheral zone of the ceramic element to becooled or heated by the flowing media.

Liquid nitrogen is suitable in a particularly advantageous way as themedium for cooling the peripheral zone, whereby the liquid film can bemade to solidify in its peripheral zone.

A device produces particular advantages if hot water is used as themedium for heating the peripheral zone, as a result of which theperipheral zone of the liquid film thaws again, it being particularlyappropriate for the hot water to be drained off from the cooling circuitof the waste process heat.

The device can be operated advantageously if the liquid film consists ofan aqueous solution, in particular consists of water, it being favorableif a film of water is retained within the solidified peripheral zone.

A device is also advantageous if at least the ceramic element is adaptedto the material of the wafer in terms of its thermal expansion behavior.

Also advantageous is the cost saving in respect of materials andinvestments and equipment, since there are no protective films orpeeling films and no investments for laminating and peeling devices arerequired.

In addition, there are savings in respect of operator tasks, and also inrespect of the provision of clean room areas, since the treatment can becarried out in classes of clean room that are subject to lowerrequirements, for example, more than 10 particles with a diametergreater than 0.1 micrometer per 28 l (1 ft³) of air.

Furthermore, with the same throughput in the grinding machine, lowerprocessing times in pre-assembly can be realized.

Furthermore, the risk of rupture is reduced as a result of reduced waferhandling.

Wafer bowing is less as a result of reduced Si oxide growth.

The method according to an embodiment of the invention and the devicefor carrying out the method can be used for all products such as memory,logic or power wafers with back-side thinning, in particular, also forwhat are known as bumped wafers.

An embodiment of the invention is also explained in more detail in thedrawings on the basis of an exemplary embodiment of a device for fixinga wafer.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of embodiments of the presentinvention, and the advantages thereof, reference is now made to thefollowing descriptions taken in conjunction with the accompanyingdrawing, in which:

FIG. 1 shows a greatly simplified section through an assembly carrier;and

FIG. 2 shows a plan view of the assembly carrier according to FIG. 1.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

According to the representation in FIG. 1, an assembly carrier 1substantially comprises a vacuum table 2, on the surface of which thatis facing the wafer 3 to be fixed there is a porous ceramic element 4.Running in the ceramic element 4 are channels 5, which serve for theheating or cooling of the ceramic element 4. Between the ceramic element4 and the wafer 3 to be fixed there is a liquid film 6, formed from anaqueous solution, onto which the wafer 3 is placed with its front sideand to a certain extent floats. The front side of the wafer 3 is to beunderstood as meaning that side of the surface on which the electriccircuits are located. As a result of the process-related application ofthe circuits, the front side of the wafer 3 has relatively pronounceddifferences in height. As already described at the beginning, inparticular, in the case of more recent forms of mounting forsemiconductor chips, for example, for flip-chip mounting, additionalmetal connectors, known as bumps, with a height of up to, e.g., 150 μmare applied on the front side of the wafers, serving for directcontacting on printed circuit boards. Even these relatively extremedifferences in height on the front side of the wafer 3 are equalized bythe liquid film 6, since the liquid film 6 uniformly encloses thetopographies.

The fixing is not then performed by vacuum chucking of the wafer 3 byway of the ceramic element 4, as in the prior art, but by targetedcooling of the ceramic 4 with the aid of liquid nitrogen, which flowsthrough the channels 5. In this case, the ceramic 4, covered by theliquid film 6, is cooled in its peripheral zone 7 to the extent thatthere is peripheral icing 8 of the liquid film 6, which fixes the wafer3 at its periphery. In order to make the icing possible it isadvantageous to use an aqueous solution or water as the liquid film 6,which provides considerable cost advantages.

As a result of the peripheral icing 8, retained in the inner region ofthe peripheral icing 8 is a film of water 6, which bathes thetopographies of the components and even the bumps, takes up and possiblyabsorbs the mechanical pressure peaks and vibrations during the grindingof the back side of the wafer 3 and keeps the front side of the wafer 3free from exposure to any constraining forces. In this case, as a resultof the hydrostatic pressure in the film of water 6, the grinding forcesoccurring from above during the back-side grinding of the wafer 3 aretaken up and the wafer 3 is relieved.

If required, it is possible on account of the surface tension of theliquid film 6 for the wafer 3 to be adapted to a slightly conical shapeof the chuck if—as in some cases—its center lies 40 μm higher than itsperipheral region 7.

What is claimed is:
 1. A device for processing wafers, the devicecomprising: an assembly carrier; a dispenser for applying a liquid filmto a region of the assembly carrier, the region including a peripheralzone of the assembly carrier and a central zone surrounded by theperipheral zone; and a temperature-controlling device adapted to controla temperature of at least the peripheral zone of the assembly carrier,wherein the temperature-controlling device is configured to change astate of the liquid film in the peripheral zone of the assembly carrierbut not the central zone surrounded by the peripheral zone.
 2. Thedevice as claimed in claim 1, wherein the assembly carrier comprises avacuum table that has a ceramic element for receiving a wafer, thetemperature-controlling device controlling the temperature of at least aperipheral zone of the ceramic element.
 3. The device as claimed inclaim 2, further comprising a channel in the peripheral zone of theceramic element such that a fluid can flow in the channel.
 4. The deviceas claimed in claim 3, wherein the peripheral zone of the ceramicelement is cooled by a fluid flowing through the channel.
 5. The deviceas claimed in claim 3, wherein the peripheral zone of the ceramicelement is heated by a fluid flowing through the channel.
 6. The deviceas claimed in claim 1, further comprising a liquid film disposed at theregion of the assembly carrier.
 7. The device as claimed in claim 6,further comprising liquid nitrogen disposed adjacent the peripheral zonesuch that portions of the liquid film adjacent the liquid nitrogen aresolidified.
 8. The device as claimed in claim 6, further comprising hotwater disposed adjacent the peripheral zone such that portions of theliquid film adjacent the hot water are liquefied.
 9. The device asclaimed in claim 8, further comprising a drain such that the hot wateris drained off from the assembly carrier during treatment of the wafer.10. The device as claimed in claim 6, wherein the liquid film includes,an annular peripheral zone of the liquid film that is in a solid state.11. The device as claimed in claim 6, wherein the liquid film comprisesan aqueous solution.
 12. The device as claimed in claim 1, wherein thetemperature-controlling device is configured to change a state of aliquid film applied to a front side of a wafer disposed over theassembly carrier in such a way that the liquid film can be made tosolidify and the wafer can be fixed to the solidified liquid film. 13.The device as claimed in claim 1, wherein the assembly carrier comprisesa vacuum table that has an element for receiving a wafer, the elementbeing formed of a material that matches a thermal expansion property ofthe wafer.
 14. A device for processing wafers, the device comprising: avacuum table; a ceramic element disposed over the vacuum table andconfigured to receive a wafer; a channel disposed in a peripheral zoneof the ceramic element and configured to flow a fluid around the wafer;a dispenser adapted to apply a liquid film to a region of the ceramicelement, the region including the peripheral zone and a central zonesurrounded by the peripheral zone; and a temperature-controlling deviceadapted to control a temperature of the peripheral zone by controllingfluid flow through the channel, wherein the temperature-controllingdevice is configured to change a state of the liquid film in theperipheral zone but not the central zone surrounded by the peripheralzone.
 15. The device of claim 14, wherein the temperature-controllingdevice is configured to solidify a portion of the liquid film around theperipheral zone.
 16. The device of claim 15, wherein thetemperature-controlling device is configured to leave a remainingportion of the liquid film as a liquid.
 17. The device of claim 14,wherein the fluid is a coolant.
 18. The device of claim 17, wherein thecoolant is liquid nitrogen.
 19. The device of claim 14, wherein thefluid is hot water.